1. Field of the Invention
The present invention relates to a method for preparing a silicon nitride ceramic with high strength and toughness and, more particularly, to a method for preparing a silicon nitride ceramic with high strength and toughness which has a double microstructure preferable for enhancing mechanical properties, by adding carbon powder as a reducing agent to silicon nitride (Si.sub.3 N.sub.4) powder containing a sintering agent such as yttria and alumina, milling the mixture of carbon powder and silicon nitride powder and controlling the oxygen content in a liquid phase obtained after a carbothermal reduction treatment of a molding at a specified temperature.
2. Discussion of Related Art
Silicon nitride (Si.sub.3 N.sub.4) ceramic is generally utilized in the whole field of industry owing to its harmonized excellent properties such as high-temperature strength, chemical stability and abrasion resistance. Especially, silicon nitride (Si.sub.3 N.sub.4) ceramic is known as a favorable material practical to heat engines such as a gas turbine thanks to its good properties at a high temperature.
But, the silicon nitride ceramic is inferior to the other metallic materials in regards to fracture toughness and reliability. After many studies made to enhance those properties of the silicon nitride ceramic, it is found that the silicon nitride ceramic can have its strength and toughness enhanced when it is controlled to have a double microstructure containing crystals distributed in rough gas particles.
Such a silicon nitride ceramic with the microstructure can have its strength and fracture toughness enhanced because of crack bridging formed by the rough hexagonal rod type crystals.
It is therefore important that the effective double microstructure has large-sized hexagonal rod type crystals distributed in a gas phase made up of fine particles. These fine particles whose usefulness depends on the particle size of the starting material powder can be obtained by reducing the particle size of the material powder by means of high-energy milling. The rough crystals are produced by addition of .beta. silicon nitride (.beta.-Si.sub.3 N.sub.4) seeds.
However, high-energy milling of the material powder increases the specific surface area of the powder and hence causes an increase in the amount of silicon oxide SiO.sub.2 on the surface of the silicon nitride particles. This affects densification of silicon nitride and phase transition so that lots of liquid phase and silicon oxynitride (Si.sub.3 N.sub.2 O) are formed, thereby influencing the properties of silicon nitride at a high temperature.
It is therefore necessary to reduce the excessive oxygen content in silicon nitride powder through an appropriate post treatment such as carbothermal reduction treatment after the high-energy milling in order to have an appropriate liquid phase with densification and good properties of the silicon nitride.
The carbothermal reduction treatment is a most widely used method for preparing highly pure silicon nitride powder from low-price materials. In the carbothermal reduction treatment, an appropriate amount of carbon is added to silica (SiO.sub.2) powder and a heat treatment is performed under the atmosphere of nitrogen gas at an appropriate temperature. Then, silicon nitride powder is produced according to a chemical reaction expressed by: EQU 3SiO.sub.2 (s)+6C(s)+2N.sub.2 (g).fwdarw.Si.sub.3 N.sub.4 (s)+6CO(g) (1)
In a case where the above method is used for fine silicon nitride powder containing a lot of sintered preparation, the oxygen content in the liquid phase can be reduced according to the temperature for the carbothermal reduction treatment and the composition and the amount of the liquid phase are dependent upon the amount of carbon added, and temperature and time for the heat treatment, etc.
There are many reports on how the addition of carbon affects the sintering of the silicon nitride. Wotting and his coworkers prepare silicon nitride powder containing a small amount of carbon through a carbon reduction treatment and sinter the silicon nitride preparation. In this experiment, the silicon nitride powder becomes harder to sinter with an increase in the carbon content, thereby resulting in a fine structure not preferable for enhancement of mechanical properties. It is reported that the result is because the carbon added reacts with silica (SiO.sub.2) or oxygen on the surface of the silicon nitride powder to reduce the amount of liquid phase and change the composition. Watari and his coworkers report that carbon affects the phase transition of carbon-coated silicon nitride powder subjected to hot pressure (HP) sintering and hot iso-pressure sintering.
It is also reported that the remaining carbon forms silicon carbide (SiC) at a high temperature to affect the high-temperature sintering adversely.
An addition of the sintering agent is necessary for complete densification because the silicon nitride is formed by strong covalent bonds with a low self-diffusion coefficient. The liquid phase formed by the reaction between the sintering preparation added and silica (SiO.sub.2) on the surface of the silicon nitride promotes the sintering property.
Examples of the sintering agent include metal oxides such as Y.sub.2 O.sub.3, Al.sub.2 O.sub.3 and MgO. Especially, for a Si.sub.3 N.sub.4 --Y.sub.2 O.sub.3 --Al.sub.2 O.sub.3, the amount and the properties of the Y--Si--Al--O--N glass phase produced greatly affect the mechanical properties of silicon nitride ceramic such as high-temperature strength and crip resistance. There have been made many studies on this effects.
An Y--Si--Al--O--N glass phase which is produced when N is replaced with O in the Y--Al--Si--O glass phase has its properties variable according to the ratio of N to O. With an increase in the N content, oxynitride crystals can be deposited due to solid threshold in the glass phase. It is also reported that there are increased glass transition temperature, viscosity and elastic coefficient since the Si--N bond is shorter than the Si--O bond.
As understood from the phase property of the silicon nitride, two phases of silicon oxynitride (Si.sub.3 N.sub.2 O) appear in a case of high oxygen content and only silicon nitride (Si.sub.3 N.sub.4) appears when using a general sintering agent (5.0-6.0% Y.sub.2 O.sub.3 +1.0-2.0% Al.sub.2 O.sub.3). With a reduction of oxygen content, there can be obtained two phases of apatite (Y.sub.10 Si.sub.3 O.sub.24 N.sub.2), woolastonite (YSiO.sub.2 N), wohlerite (Y.sub.4 Si.sub.2 O.sub.7 N.sub.4) and melilite (Y.sub.2 Si.sub.3 O.sub.3 N.sub.4).
When the apatite phase is formed during the sintering, the amount of liquid phase decreases to inhibit phase transition and sintering but rapidly increases during the sintering to produce a unique fine structure at a temperature exceeding the melting point of the apatite phase, 1750.degree. C.
Although particle rearrangement occurs in general with a rapid increase in the density due to the liquid phase formed at around 1400.degree. C., it is expected that the particle rearrangement is very difficult to take place in the above case due to formation of apatite and occurs rapidly at above the melting point of the apatite, 1700.degree. C. In such a case, the boundary between the crystalline particles and non-crystalline particles is much prominent.
In the related art method for preparing a silicon nitride ceramic, the liquid phase formed by reaction between the sintering agent added and silica (SiO.sub.2) on the surface of the silicon nitride powder causes sintering. It is therefore required to control the fine silicon nitride particles to have a double microstructure in which large-diameter rod-type reinforcing silicon nitride particles uniformly distributed in a gas phase, in order to enhance strength and toughness of the silicon nitride ceramic.
For this purpose, about 2-5% of silicon nitride seeds several micrometers in diameter are added to induce growth of reinforcing crystals.
However, such a related art method is disadvantageous in that the preparation of the silicon nitride seeds is too complicated and takes too much time.